On some occasions it is convenient or essential to be able to convert one DC voltage to another. For instance, in digital systems it is common to have +5 volts available at high current nearly everywhere, but it may be desirable to operate a single operational amplifier that needs only a few milliamps of .+-.15 volts DC. Or it may be desirable to operate some high-power equipment from a 12 volt automobile storage battery.
For DC sources of voltage, power supplies may employ DC to DC converters to convert unregulated DC voltage to regulated or controllable DC voltage. A typical converter utilized is the Pulse Width Modulated (PWM) Converter. These converters employ square-wave pulse width modulation to achieve voltage regulation by varying the duty cycle of the waveform. However, typical PWM converters can handle 40 VDC maximum.
A fluorescent lamp power supply is disclosed in U.S. Pat. No. 4,920,302, issued to Konopka. The power supply includes a converter for supplying power from a low voltage DC voltage source for operating a pair of fluorescent lamps. The on-time of the converter is controlled by a pulse width modulator circuit comprising dual comparators. A reference potential is applied to a potentiometer for controlling the output comparator conduction threshold and for varying the width of the pulses of the pulse width modulator. Variation of the reference potential changes the brightness of the fluorescent lamp. The system disclosed utilizes pulse width modulation to control the voltage output, however, it converts relatively low DC input voltages to high AC voltages.
A system for controlling the current supplied from a DC power source to high power illuminating lamps is disclosed in U.S. Pat. No. 4,450,384, issued to Krokaugger. The current fed to the lamp is controlled by manually setting the width of the pulsed output of a pulse width modulator by means of a potentiometer connected to the pulse width modulator. The power for the DC controller, which includes the pulse width modulator, is provided from the same DC power source and stepped down by a DC--DC converter. The system disclosed fails to describe the operation of the voltage step-down DC--DC converter. In addition, the brightness of the lamp requires manually setting the DC controller.
Although DC to DC converters are available as standard electronic devices, the maximum DC input voltage that can be applied to these commercial products may be 40 VDC. Therefore, power supplies are available that convert up to 40 VDC maximum to a lower regulated DC output voltage. A problem of the prior art is that commercial products are not available that encompass the substantially wide range of input voltages of the present invention.